Heterosexual transmission accounts for the majority of new cases of HIV-1 infection each year. Recent reports from the World Health Organization estimate that a total of 38 million people are now infected with HIV-1 worldwide; 90% of whom live in developing countries. As of 2003, nearly 50% of all infected individuals were women. The increased incidence of HIV-1 infection in women, particularly those of childbearing age, underscores the urgent need for effective preventative and therapeutic options that are safe, affordable and culturally accepted. In the absence of an effective preventative vaccine, topical microbicides may offer a practical alternative to block HIV-1 transmission at the site of entry. We seek funds to determine whether the process of RNA interference (RNAi) can be harnessed to prevent the cervicovaginal transmission of HIV-1 and thereby emerge as a novel microbicidal strategy. This work will be conducted by investigators at Rhode Island Hospital (B. Ramratnam) and Harvard-NEPRC (K. Mansfield). RNAi refers to the sequence-specific degradation of RNA that follows the cellular introduction of homologous, short interfering (si) RNA. We hypothesize that RNAi can be specifically targeted to the vaginal/rectal mucosa and decrease the expression of host factors that mediate HIV-1/SIV mucosal transmission. For these proof-of-principle studies, we will target the CCR5 chemokine receptor in macaques. We will pursue two specific aims over the 24-month period. 1: To characterize the tissue penetrance of liposomal siRNA following direct vaginal/rectal application. 2: To quantify the kinetics and durability of mucosal CCR5 knockdown following single and serial vaginal/rectal applications of siRNA. At the end of the 12-month period, we will have defined the HIV-1 microbicidal potential of siRNA. The full realization of this potential will come from more extended experiments involving vaginal/rectal virus challenge in CCR5 siRNA treated macaques and appropriate controls.